U.S. Secretary of State Mike Pompeo on Friday lambasted his predecessor, John Kerry, for meetings with Iranian officials in back-channel talks and accused him of “actively undermining” the Trump administration’s policy toward Tehran.

“What Secretary Kerry has done is unseemly and unprecedented,” Pompeo told a news conference, adding that he “ought not to engage in that kind of behavior. It’s inconsistent with what the foreign policy of the United States is, as directed by this president. It is beyond inappropriate.”

Pompeo’s sharp criticism of Kerry comes a day after President Donald Trump accused the former secretary of state of “illegal” meetings on Iran in a late-night tweet.

“John Kerry had illegal meetings with the very hostile Iranian Regime, which can only serve to undercut our great work to the detriment of the American people. He told them to wait out the Trump Administration! Was he registered under the Foreign Agents Registration Act? BAD!” Trump said on Twitter.

Trump withdrew the United States from a nuclear deal that Kerry clinched in 2015 between Iran and six world powers. The Trump administration has pushed a hard line against Tehran, which it accuses of expanding its influence in the Middle East through support for proxy armed groups in countries such as Syria, Lebanon and Yemen.

Kerry, in a radio interview with Fox News as part of a book tour, said he had met with Iranian Foreign Minister Mohammad Javad Zarif “three or four times” since the end of his term in January 2017. He has also accused the Trump administration of pursuing a policy of regime change in Iran.

“Mr. President, you should be more worried about Paul Manafort meeting with Robert Mueller than me meeting with Iran’s FM,” Kerry said on Twitter shortly after Pompeo’s remarks, referring to Trump’s former campaign chairman who on Friday agreed to cooperate with federal investigators into Russian meddling in the 2016 U.S. election.

“There’s nothing unusual, let alone unseemly or inappropriate, about former diplomats meeting with foreign counterparts,” a spokesman for Kerry said in a statement. “What is unseemly and unprecedented is for the podium of the State Department to be hijacked for political theatrics,” he said.

During interviews this week Kerry has accused the Trump administration of pursuing a policy of regime change in Iran.

Pompeo pushed back, saying that was not the administration’s intention.

“This is a former secretary of state engaged with the world’s largest state sponsor of terror ...,” Pompeo said, adding: “He was telling them to wait out this administration. You can’t find precedent for this in U.S. history.”